This invention relates generally to devices used in manufacturing assemblies from metal pipe and tubing. More particularly, the invention relates to a jig for use in fitting up and joining header assemblies used in heat exchangers. The jig includes an integral gas purging capability.
A header, sometimes called a manifold, is a conduit into which a plurality of other conduits open. Headers are widely used in a variety of applications, particularly in the air conditioning and refrigeration industry. A typical application in that field is in conjunction with tube type heat exchangers. In order to reduce the total pressure drop across a given heat exchanger, the refrigerant flow through the heat exchanger may be split into two or more parallel flow paths, or circuits. This is done by installing an inlet header and an outlet header on the upstream and downstream sides of the heat exchanger, respectively. The inlet header receives a flow of refrigerant from a single conduit and distributes the flow to the various parallel flow paths in the heat exchanger. The outlet header receives the flow from the parallel flow paths and directs a single flow of refrigerant to components downstream of the heat exchanger.
Tube type heat exchangers used in air conditioning and refrigeration applications are generally made of copper pipe or tube. Headers used in the same applications are also generally made of copper pipe or tube. Such a header comprises a pipe or tube of a relative larger diameter having smaller diameter tubes that join into it. In a typical manufacturing operation for the large scale production of heat exchangers, header assemblies are usually prefabricated with short nipples, or stubs, of tube of an appropriate diameter extending from the header at appropriate positions and orientations. The prefabricated header assembly is then assembled to its heat exchanger by joining the nipples to the heat exchanger tubing to form the parallel flow paths.
Tube nipples are generally joined to a header by insertion into appropriately prepared apertures in the ends or side-wall of the header tube. The nipples are then joined to the header by a process such as soldering, brazing or welding to form a joint that is both fluid tight and mechanically sound. There is usually a shoulder formed in or attached to a nipple at an appropriate distance from the end to be inserted into the header in order to improve the quality of the finished joint and to assure the correct insertion distance.
If the nipple is fitted up and joined to the header manually by an assembly worker, as is frequently the case, the nipple alignment is dependent on the skill of the worker. Even slight misalignment of a nipple can complicate assembly of a header to a heat exchanger. One technique for attaining the proper fit in a finished header is to bend the nipples to the correct alignment using, for example, an aligning jig. But such a procedure is labor intensive, can subject the nipples and the nipple joint to excessive stress and introduces another step into the heat exchanger assembly process. Manual joining of nipples to the header is in itself time consuming and labor intensive.
In order to properly braze a joint, the metal surrounding the joint must be heated to a relatively high temperature. In a copper header and nipple assembly of the size and configuration used in the typical air conditioning or refrigeration heat exchanger, the entire assembly will be subjected to high temperatures. At high temperatures and in the presence of oxygen, copper will oxidize. Thus copper oxides may form on and in the header assembly during brazing, either as loose particles or as an oxide coating on the tube walls. A brazed header assembly must be free of copper oxides before it is installed into a refrigeration system, for loose particles may cause damage to other system components, e.g. the compressor. It is not sufficient to remove just the loose particles, for particles may become detached from the coating some time after header assembly to contaminate the system. The interior of header assemblies can be cleaned to remove the oxides of copper, but the cleaning process is difficult and expensive.
It is axiomatic that, even at high temperatures, in the absence of oxygen, copper will not oxidize. Oxygen can be excluded from contact with hot copper by blanketing the surfaces with a suitable gas. Thus, a gas purge of the interior of a header assembly during brazing of the nipples will prevent the formation of copper oxides, leaving bright interior surfaces that do not require cleaning. Such a purge must, however, blanket the entire internal surface of the header assembly and therefore there must be provisions to insure that the purge gas can reach all interior areas of the header assembly.